They leveled personal attacks on issues ranging from immigration to foreign policy to abortion as the race intensified ahead of the Palmetto State primary next Saturday.
They repeatedly interrupted and talked over one another, and made disparaging comments. There were duels between billionaire businessman Donald Trump and Texas Sen. Ted Cruz, between Cruz and Florida Sen. Marco Rubio, between former Florida Gov. Jeb Bush and Trump. Ohio Gov. John Kasich, meanwhile, repeatedly criticized the tone on the stage and gambled that enough South Carolina Republicans would be turned off by what they heard to provide an opening for him.
Forget the governors' alliance. File away that Trump-Cruz detente. All those debate-stage friendships are over.
Sticking to South Carolina's tradition of bare-knuckled politics, Saturday night's debate featured the most bitter exchanges of the campaign so far. The audience was equally vocal, and the atmosphere indicated that the negative -- and personal -- attacks will only increase as the race drags on.
In one of the most memorable exchanges of the night, Donald Trump unloaded on his former friend Ted Cruz.
"You are the single biggest liar," he said of his former friend Cruz.
Cruz had just accused Trump of supporting government funding for Planned Parenthood, and Trump had shot back that Supreme Court Chief Justice John Roberts' decisions (like upholding Obamacare) are Cruz's to bear.
At one point Trump attacked the Texas senator by saying: "This guy will say anything. Nasty guy. Now I know why he doesn't have one endorsement from any of his colleagues. He's a nasty guy."
As evidence, Trump pointed to the Cruz campaign's tactics in Iowa, where aides asserted -- inaccurately -- that former neurosurgeon Ben Carson was dropping out of the race while voters were still making decisions at the state's caucuses.
Cruz shot back: "It's fairly remarkable to see Donald defending Ben after he called him pathological and compared him to a child molester. ... Donald has a weird pattern. When you point to his own record, he screams, 'liar, liar, liar.'"
2. Rubio vs. Cruz, en español
Speaking of attacks: The race's two freshman senators of Cuban heritage, Florida's Marco Rubio and Texas's Ted Cruz, really don't like each other.
Their exchanges over immigration are at this point almost a Republican debate tradition.
But there was a new element Saturday night: Cruz shouted at Rubio -- in Spanish.
Cruz had accused Rubio of saying on Univision in Spanish that he would not repeal President Barack Obama's executive actions on immigration.
Rubio retorted: "I don't know how he knows what I said on Univision because he doesn't speak Spanish."
That's when Cruz looked at Rubio and uttered a few words in Spanish, challenging Rubio to continue in that language "if you want."
The battle was over the same immigration issues that have been the subject of attacks between the two for months: Rubio sponsored an immigration overhaul measure in 2013 that would have created a path to citizenship for undocumented immigrants. Cruz opposed it, but offered his own amendment swapping out a path to citizenship for legal status. Rubio has repeatedly raised it to muddy their differences on immigration.
"Ted Cruz has just been telling lies," Rubio said.
Though each one scored hits as they battled each other during the debate, Rubio turned in a noticeably better performance than he did last time -- when he faltered in the days leading up to the New Hampshire primary, which he partly blamed for his fifth-place finish.
Throughout that exchange and the entire night, Rubio was aided by New Jersey Gov. Chris Christie's decision to drop out of the race. It was Christie's battering that led the Florida senator to stumble in the last debate.
On Saturday, Bush stepped in to play that role, saying: "I feel like I have to get into my inner Chris Christie." But unlike Christie, the debate moved on and he dropped the matter.
3. Battling over Bush
South Carolina was where Bush and Rubio -- the teacher and the student in their Florida statehouse days -- were supposed to go head-to-head.
Instead, the two of them teamed up against another opponent, Donald Trump.
What got them going: Trump blamed George W. Bush for the September 11, 2001, terrorist attacks.
The two mounted a fierce defense of the 43rd president's handling of national security, while Trump trashed him. That's a risky move, given that angering South Carolina's military-oriented electorate could endanger his big lead in the polls.
"Obviously the war in Iraq was a big, fat mistake," Trump declared in an encounter on foreign policy.
"I am sick and tired of him going after my family," Bush shot back. "While Donald Trump was building a reality TV show, my brother was building a security apparatus to keep us safe."
That's when Trump hit even harder.
"I lost hundreds of friends. The World Trade Center came down during the reign of George W. Bush," Trump said. "He kept us safe? That is not safe."
Rubio jumped in and defended Bush 43, blaming another president -- Bill Clinton -- for failing to take Osama bin Laden out "when he had a chance."
Trump's line of attack drew boos from the audience. But Trump didn't seem to mind. He dismissed a chorus of boos as coming from Jeb Bush's donors.
4. Kasich's Achilles heel
Kasich's single biggest weakness with Republican primary voters has long been his decision to expand Medicaid in Ohio under Obama's health care law.
His second-place finish in New Hampshire finally had Kasich -- who prefers to stay out of the fray -- playing defense on that decision.
The attack came from Bush, who said: "We should be fighting Obamacare, repealing Obamacare, replacing it with something totally different."
Kasich made his case in moral terms, aligning himself with former President Ronald Reagan, the conservative hero who, the Ohio governor said, "expanded Medicaid five times," arguing that the government-run insurance program helps poor people transition to productive work.
He also showed he's been studying the Bush opposition research book, saying that in the former Florida governor's first four years, "his Medicaid program grew twice as fast as mine, okay? It's just a fact."
"He knows that I'm not pro-Obamacare, never have been," Kasich said.
Kasich doesn't often mix it up with his debate-stage rivals. All the fighting, he said, has the GOP "fixin' to lose the election."
But he could find himself increasingly under scrutiny if he's able to prove in South Carolina that his appeal to moderates and establishment-type Republicans extends beyond New Hampshire.
5. Replacing Scalia
Hours after the news broke that Scalia had died, the fight over when to replace the conservative judicial figure quickly took shape as a defining issue of the 2016 presidential race.
The Republican presidential field thinks one of its members -- not Obama -- should pick Scalia's successor.
Trump said Senate Republicans should "delay, delay, delay." Cruz agreed, saying that "the Senate needs to stand strong and say we're not going to give up the Supreme Court for a generation."
Bush didn't call on Obama to skip nominating a replacement, but said it should be a "consensus" pick -- unlikely in today's partisan environment.
It was the one issue over which Republicans expressed broad consensus, focusing their attacks on Democrats. (In fact, the audience put its finger on the scale -- not to support a candidate, but to boo CBS moderator John Dickerson after he'd fact-checked Cruz's inaccurate assertion that no Supreme Court nominee has been confirmed in an election year in the past 80 years. Anthony Kennedy was confirmed on February 3, 1988, during Ronald Reagan's final year in office.)
The comments from the GOP field came just after Obama had said he does, indeed, intend to nominate a replacement for Scalia -- and Senate Majority Leader Mitch McConnell had said he wouldn't give that nominee a vote.
In Denver, Hillary Clinton had blasted McConnell's stance as "totally out of step with history and constitutional principles."